When a sample of a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0, defining the Z axis of a Cartesian coordinate system), the individual magnetic moments of the spins in tissue nuclei attempt to align with this polarizing field, but precess about it in random order at the characteristic Larmor frequency of the nuclei. If the substance is subjected to a magnetic field (excitation field B.sub.1) which is in the X-Y plane of the Cartesian coordinate system and which oscillates near the Larmor frequency, the net magnetic moment of the sample aligned along the Z axis, M.sub.z, may be rotated, or "tipped", into the X-Y plane to produce a net transverse magnetic moment M.sub.t. A magnetic resonance signal is emitted by the excited sample after the excitation signal B.sub.1 is terminated. This signal may be modified by application of additional magnetic fields and may be received and processed to form an image. This process known as magnetic resonance imaging (MRI) may be used for medical diagnosis as well as non-medical purposes.
When utilizing these magnetic resonance signals to produce images, magnetic field gradients (G.sub.x, G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is subjected to a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The polarizing field B.sub.0 and the magnetic field gradients (G.sub.x, G.sub.y, and G.sub.z) typically are produced by relatively large electromagnetic coils around the patient being imaged. The B.sub.1 field may be transmitted into the object to be imaged by an antenna, which may be large or small compared with the object, and may or may not also be used to receive the subsequent magnetic resonance signal. Much smaller receive-only antennae, known as "local coils" or "surface coils", commonly are placed in close proximity to the portion of the patient to be imaged in order to better receive the magnetic resonance signals. The resulting set of received signals are digitized and processed to create the image using one of many well-known reconstruction techniques.
Both the transmit and receive antennae are normally resonant at the frequency which is determined by the nuclear species and the static magnetic field strength, to maximize their efficiency. Unfortunately, these antennae can interact with each other via inductive and/or capacitive coupling, changing each other's effective resonant frequency and distorting each other's radio frequency field spatial distribution. This is ordinarily prevented by disabling the receive antenna during the transmit phase of the imaging process, and disabling the transmit antenna resonance during the receive phase.
FIG. 1 shows a technique for enabling and disabling an MRI antenna with series RF switching using a PIN diode activated by DC current pulses. The antenna circuit consists of inductance 10 in series with capacitances 13 and 14 and PIN diode 15, creating a series resonance which has a peak response at the resonant frequency, selected to be the Larmor frequency of the sample nuclei. Capacitance 13 may be a discrete capacitor, a plurality of discrete capacitors in series, or a distributed capacitance. Inductance 10 normally is not a discrete inductor but the distributed inductance of a conducting structure, such as a loop of wire, which receives the magnetic resonance signal. Capacitance 14 is normally a discrete capacitor chosen to provide an impedance match between the antenna circuit and the transmission line, consisting of conductors 11 and 12, which carries the RF signal to the system's receiver. As illustrated the antenna circuit is completed when a DC current 17 is sent through the PIN diode 15, causing PIN diode 15 to conduct at RF frequencies. PIN diodes are utilized because of their high on/off conductance ratio. The circuit is open (disabled) when DC current is not applied to the PIN diode 15. This approach is less than optimum due to significant RF signal loss in PIN diode 15.
In the alternative circuit shown in FIG. 2 the series-resonant antenna circuit consists of inductance 10 and capacitances 18 and 22. Inductance 16, capacitance 18, and PIN diode 20 form a blocking resonant loop coupled through capacitor 18 to the antenna circuit. The DC disable signal is applied to terminals 24 and 26, which also serve as the RF signal terminals. Note that the polarity of PIN diode 20 must be such that the DC disable signal produces forward current through the diode. When DC current flows through PIN diode 20 inductance 16 is placed in parallel with capacitance 18, creating a parallel resonance which has a minimum response at the resonant frequency, selected to be the Larmor frequency of the sample nuclei. By adjusting inductance 16 it is possible to substantially null the response of the antenna circuit at the resonant frequency. This disables the antenna and minimizes its effect on the other antennae in the MRI system. In this embodiment, the antenna's resonant circuit does not include PIN diode 20 and thus the loss of that diode does not produce unwanted signal loss.
When transmit or receive antennae are placed in close proximity to the patient; they present a potential source of hazardous DC or low frequency AC voltage and current in the event that one or more components fail. The potential for electric shock has become of greater concern with the advent of interventional procedures performed within an MRI system. In such procedures conductive fluids such as normal saline and body fluids from the patient often are present, and it may not be possible to guarantee complete isolation of these fluids from the conducting components associated with the antenna.
For safety and shock prevention, it is possible to electrically isolate the RF signal path by AC coupling the system receiver to the antenna. When blocking techniques such as that shown in FIG. 2 are used, the blocking loop must be kept physically small to avoid inducing RF currents which can turn off or adversely heat the PIN diode 20. PIN diode 20 and inductor 16 must therefore be located at the antenna, so a DC current path must exist from the system electronics to the antenna and complete isolation is not possible.